Chloroquine Sensitizes Nasopharyngeal Carcinoma Cells but Not Nasoepithelial Cells to Irradiation by Blocking Autophagy

BACKGROUND Treatment of nasopharyngeal carcinoma requires the application of high dosages of radiation, leading to severe long-term complications in the majority of patients. Sensitizing tumor cells to radiation could be a means to increase the therapeutic window of radiation. Nasopharyngeal carcinoma cells display alterations in autophagy and blockade of autophagy has been shown to sensitize them against chemotherapy. METHODS We investigated the effect of chloroquine, a known inhibitor of autophagy, on sensitization against radiation-induced apoptosis in a panel of five nasopharyngeal carcinoma cell lines and a SV40-transformed nasoepithelial cell line. Autophagy was measured by immunoblot of autophagy-related proteins, immunofluorescence of autophagosomic microvesicles and electron microscopy. Autophagy was blocked by siRNA against autophagy-related proteins 3, 5, 6 and 7 (ATG3, ATG5, ATG6 and ATG7). RESULTS Chloroquine sensitized four out of five nasopharyngeal cancer cell lines towards radiation-induced apoptosis. The sensitizing effect was based on the blockade of autophagy as inhibition of ATG3, ATG5, ATG6 and ATG7 by specific siRNA could substitute for the effect of chloroquine. No sensitization was seen in nasoepithelial cells. CONCLUSION Chloroquine sensitizes nasopharyngeal carcinoma cells but not nasoepithelial cells towards radiation-induced apoptosis by blocking autophagy. Further studies in a mouse-xenograft model are warranted to substantiate this effect in vivo.